Various types of pressure sensing systems for monitoring the pressure within the tires of an automotive vehicle have been proposed. Such systems generate a pressure signal using an electromagnetic (EM) signal which is transmitted to a receiver. The pressure signal corresponds to the pressure within the tire. When the tire pressure drops below a predetermined pressure, an indicator is used to signal the vehicle operator of the low pressure. Many vehicles require different tire pressures in the front of the vehicle and the rear of the vehicle. Therefore, it is important to know the relative position of the pressure sensor and thus the tires relative to the vehicle. Known systems provide manual means for programming the relative positions. For example, a magnet is positioned manually near the tire to allow the system to recognize the position of the tire. Such systems rely on the vehicle operator performing the recognition in a particular order. Such systems, however, are prone to errors.
Other systems use a device attached to the wheel well that forces the sensor contained in the tire to transmit immediately. A central receiver is used to “hear” the sensor's response to the forcing operation. An initiator is the device used to force the response. The receiver finalizes the auto location by associating the sensor identification with the specific wheel location for which the response is forced.
Known initiators use a low frequency magnetic field as the forcing operation. An electric coil within the sensor senses the magnetic field. Due to the nature of the magnetic fields and regulations set forth by the United States Federal Communications Commission the coil in the sensor does not have sufficient sensitivity to the magnetic field to ensure robust operation.
Current design trends in automotive vehicles provide increased tire size. One solution to the lack of sensitivity is placing the initiator closer to the wheels or tires. However, as the wheel size increases, less space is available for decreasing the distance to the tire. Also, in truck applications the initiator is typically placed far from the sensor. Placing the coil closer to the tire still has not provided adequate performance gain.
The pressure sensors are powered by batteries which are size limited to minimize cost and weight. However, the size limitation also reduces the amount of energy available to power all of the functions, including responding to the sensing of a magnetic field. In order to preserve battery energy, sensors typically do not sense the presence of a magnetic field continuously. Every so often the sensors sample voltage on an electric coil for a very short period of time. Typical time periods are that the sensor samples every four seconds for 250 microseconds. To ensure that the sensor receives the signal, the magnetic field is transmitted for extended periods of time. The problem associated with such a solution is that the United States Federal Communications Commission limits the strength of the field (currently 5.77 nT average field strength at a distance of one meter from the initiator). However, the FCC allows peak field strength to increase by up to 20dB for a device that transmits a field for less than 10ms. Another source of degradation in current solutions is the orientation of the coil in the sensor relative to the electric coil in the initiator. Ideally, the axes of these coils are parallel to ensure maximum coupling. However, the coil in the sensor may be rotated relative to the coil in the initiator as the wheel is turned to turn the vehicle. Misalignments in a locked turn can decrease the amount of power by a factor of 2 (3dB). One solution to this is adding a coil in the sensor orthogonal to the existing coil. As the wheel is turned, the second coil increases its ability to sense the magnetic field. One problem with such a solution is that the package size increases for the addition of the new coil. Because of the design restriction to provide common parts for several vehicles, vehicles that do not require such systems may incur the additional cost and mass increase.
It would therefore be desirable to provide a tire pressure monitoring system that automatically and reliably identifies the position of each tire relative to the vehicle.